How are you going to use the car? On the street or at the strip? Street tires or slicks? What rear axle ratio?

M20's and M21's were put behind a lot of high HP muscle cars in the 60's. I ran an M20 in a GTO behind a 400, bored .030, 12:1 compression, Crane hydraulic 308/316 .544/.561 (with 1.65 Ram Air V rockers), 850 double-pumper on a Tarantula manifold, mechanical advance Unilite, HD Scheifer clutch & flywheel, 4.10 posi. I ran 10" wrinklewalls on Wednesday bracket night at Cornhusker Raceway in Omaha. Launched at 7,000, shifted at 6,500. Probably 30-40 passes a year for 3 years. Flogged it a bit on the street more than a few times too. I drove the car stock for 10 years before that. Rebuilt the trans once, with all new synchros and bearings. Never broke a thing.

But, that was just me. Your mileage may vary.